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Chemical storage of hydrogen in synthetic liquid fuels: building block for CO2-neutral mobility

Chemical storage of hydrogen in synthetic liquid fuels: building block for CO2-neutral mobility Green hydrogen is anticipated to play a major role in the decarbonization of the mobility sector. Its chemical storage in CO -neutral synthetic liquid fuels is advantageous in terms of safety and reliability compared to other hydrogen storage developments, and thus represents a complementary building block to developments in electric and hydrogen mobility for the low-carbon transition in the mobility sector. Its development is especially relevant for transport sectors which will have no alternatives to liquid fuels in the foreseeable future. In this paper, three alternative technological routes for the chemical storage of hydrogen in CO -neutral synthetic liquid fuels are identified and comparatively evaluated in terms of feedstock potential, product potential, demand for renewable electricity and associated costs, efficiency as well as expected market relevance. While all three routes exhibited similar levels of overall efficiencies, electricity-based liquid fuels in Germany are currently limited by the high cost and limited supply of renewable electricity. In contrast, liquid fuels generated from biogenic waste have a constant supply of biogenic feedstock and are largely independent from the supply and cost of renewable electricity. Received: 12 November 2020; Accepted: 22 February 2021 © The Author(s) 2021. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com 180 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 181 Graphical abstract CO Biomass Biomass Biog Biogeenic nic CO CO O O O O O - -neutr n n n ne eut tral al Biog Bioge enic nic CO CO O O O O O - -neut n n n ne eut tra ral l CO CO O O O O O - -neut n n n ne eut tra ral l 2 2 2 2 2 2 2 2 2 waste waste liquid d d fuels liquid d d fuels liquid d d fuels Ci Cirrccu ular larr carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon technologies technologies technologies Electricity-basedrenewable hydrogen WE WE E- -Fuel ls s: : Biog geni ic c c- -based d synt y y hec c fuels W W W- -Fuel ls s: : Biogenic ic c- -based d synthec c fuels E E E- -Fuel ls s: : Electricit ty y y- -based d synthec c fuels with h h renewable e e e hydrogen Keywords: CO -neutral synthetic liquid fuels; renewable hydrogen; chemical storage; waste-to-fuels; electricity- based fuels; circular carbon technologies combination with fuel cells—as an integral component of Introduction its future fuel mix. Besides lowering the carbon footprint Hydrogen is a highly versatile element that can be used as in transport, it could contribute to improving energy effi- a feedstock, a fuel or an energy carrier and for storage. It ciency while at the same time reducing the EU’s depend- has numerous applications in diverse sectors ranging from ency on fossil imports, thus enhancing its energy-supply chemical production, metallurgy and energy to mobility, security [6]. In order to inform decision-makers about the and does not emit CO or pollute the environment when potential and future of hydrogen mobility, its prospects it is utilized [1]. Hence, it is anticipated to play a key role and challenges in the form of fuel-cell vehicles have been in the transformation of carbon-intensive sectors towards intensively investigated and reported in scientific litera- carbon neutrality. While hydrogen can be produced from ture (e.g. [7–10]). At the same time, alternative routes of almost all energy resources, it is currently predominantly hydrogen storage—e.g. in ammonia (NH), metal hydrides, generated from fossil fuels, i.e. natural gas and coal. Fossil- chemical hydrides, liquid organic hydrogen carriers such based hydrogen is generally referred to as ‘grey’ hydrogen as methylcyclohexane (MCH), carbohydrates or synthetic and its global production is associated with ~830 million hydrocarbons (e.g. [11–17])—have also garnered significant tons of CO per year. This is equivalent to the carbon foot- research attention. print of the UK and Indonesia combined [2]. In view of its A safe and reliable hydrogen-storage infrastructure has potential in supporting the decarbonization of industries been identified as a necessary enabling condition for the and mobility, global interest in the production, storage and public acceptance of hydrogen mobility [18]. In this con- utilization of ‘green’ hydrogen from renewable sources text, the chemical storage of green hydrogen in synthetic with a zero carbon footprint is thus growing steadily. hydrocarbons in the form of CO-neutral synthetic liquid A major sector in which green hydrogen is envisaged to fuels to substitute fossil-based transportation fuels for play a major role in decarbonization is mobility. Currently, road vehicles, aviation and shipping has a major advan- transportation accounts for approximately one-fifth of the tage compared to other hydrogen-storage developments. global CO emissions [3]. To complement the develop- This is because synthetic liquid fuels can be safely, easily ments in battery electric vehicles (e.g. [45 , ]) in lowering and reliably distributed via existing petrol station net- the CO emissions of the mobility sector, the European works as drop-in fuel or as fossil-fuel blends for use in Commission has identified green hydrogen—especially in conventional combustion engines [19]. This is not only an Ulizaon Photosynthesis Recycling Bio-H Bio-H Bio-CO C H Bio-CO x y 2 Recycling Ulizaon Photosynthesis Bio-H Bio-H C H Bio-CO x y 2 CO from atmosphere and/or biogenic pointsources C H Bio-CO x y 2 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 182 | Clean Energy, 2021, Vol. 5, No. 2 essential development for mobility sectors such as avi- via the Fischer–Tropsch processes or the two-stage syn- ation, shipping and road (freight) which will have no real thesis of gasoline/kerosene with methanol production in alternatives to liquid fuels in the foreseeable future [19]; it the first stage followed by methanol-to-gasoline/-kerosene also represents a complementary building block for elec- in the second stage—are implemented for fuel production. tric and hydrogen mobility for the road (passenger) sector. This technological route enables the chemical storage of The latter is especially relevant for existing/older road- renewable electricity via hydrogen in CO-neutral fuels. vehicle fleets as well as regions where new infrastructures This, however, is not the only technological route through for hydrogen carriers are not economically feasible. which CO -neutral synthetic liquid fuels can be produced. While growing interest in CO-neutral synthetic liquid Other alternatives also involve the chemical storage fuels has been observed in recent years, such develop- of hydrogen, albeit in bio-based CO-neutral synthetic ments have predominantly focused on electricity-based liquid fuels. fuels (e.g. [19, 20]). The potential of alternative routes has The prerequisite for the production of bio-based CO- been so far largely neglected. To address this gap, this art- neutral synthetic liquid fuels is the use of biogenic waste icle has two main objectives, namely (i) to identify alterna- materials (e.g. waste wood, forest residues) to deliver both tive technological routes for the chemical storage of green the carbon and hydrogen components required for the hydrogen in CO -neutral synthetic liquid fuels and (ii) the fuel production. Unlike CO in the case of E-Fuels, biogenic 2 2 comparative evaluation of the alternative technological waste materials not only act as a carbon source; they also routes in terms of feedstock potential, product potential, provide the chemical energy required for the fuel syn- demand for renewable electricity and associated costs, ef- thesis. Via waste gasification, the reaction between the ficiency as well as expected market relevance. biogenic waste and oxygen produces syngas  which con- We utilize a case  analysis approach for the compara- sists of a mixture of carbon monoxide, hydrogen and CO. tive evaluation to facilitate a quantification—as far as pos- Similarly to E-Fuels, syngas can be subsequently refined sible—of the parameters considered. Our investigation via different (i.e. direct or two-stage) synthesis technolo- focuses on Germany. In view of its adoption of a National gies for the production of synthetic fuels. We call such Hydrogen Strategy and the establishment of the National bio-based synthetic fuels W-Fuels—an abbreviation for Hydrogen Council to realize its ambition to become a ‘Waste-to-Fuels’ [28, 29]. As both carbon and hydrogen global leader in hydrogen technologies [21], Germany pre- components are biogenic in origin, W-Fuels produced this sents a rich case study for the analysis of the chemical way will be CO -neutral. W-Fuels thus represent a chem- storage of green hydrogen and its potential contribution ical storage for biogenic hydrogen in CO-neutral synthetic to CO -neutral mobility. Note that the intent of this art- liquid fuels. icle is not to carry out a life-cycle assessment of CO emis- During the W-Fuels production process, not all carbon sions of the different technological routes for the chemical contained in the biogenic waste materials will be con- storage of green hydrogen in CO-neutral synthetic liquid verted into W-Fuels. Carbon losses will occur along the fuels. It should be understood as a perspective article that process chain in the form of biogenic CO emissions. With presents a comparative evaluation of the diverse techno- the integration of hydrogen from renewable electricity logical routes along multiple parameters to sensitize into the process chain, such CO could be converted into readers about the potential and challenges of alternatives additional syngas [30, 31]. We call W-Fuels produced with to electricity-based synthetic fuels. the integration of additional electricity-based renewable hydrogen WE-Fuels, as they represent chemical storage for both biogenic and electricity-based renewable hydrogen 1 Alternative technology routes for in CO -neutral synthetic liquid fuels. WE-Fuels thus offer flexible chemical storage for electricity-based renewable the chemical storage of hydrogen in hydrogen, as its integration in the fuel synthesis is an op- CO -neutral synthetic liquid fuels tion and not—as in the case of E-Fuels—a must. Both carbon and hydrogen play essential roles in the pro- Fig. 1 illustrates the chemical storage of hydrogen in duction of CO-neutral synthetic liquid fuels. In recent CO -neutral synthetic liquid fuels in E-Fuels, W-Fuels years, the predominant focus by science, industry and pol- and WE-Fuels, and their integration into the natural itics has been placed on the production of electricity-based carbon cycle. Note that the technologies for conversion of fuels, i.e. E-Fuels, also known as ‘Power-to-X Fuels’ or ‘PTX- carbon and hydrogen sources into CO-neutral synthetic Fuels’. Generally, the carbon source for ‘classical’ E-Fuels liquid fuels are generally referred to as circular carbon is provided by CO from point sources and/or from the at- mosphere [19, 22]. CO , however, can not provide the chem- Please refer to [20] for an overview of E-Fuels production ical energy that is required for the fuel synthesis. Rather, processes. the chemical energy required is obtained from renewable Please refer to [23] and [24] for an overview of the gasification hydrogen that is generated via water electrolysis powered processes for syngas production. For an overview of the syn- by renewable electricity. Subsequently, standard synthesis thesis processes for fuel production from syngas, please refer to processes—either the direct synthesis of diesel/kerosene [25], [26] and [27]. Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 183 CO Biomass Biomass Biog Biogeenic nic CO CO O O O O O - -neutr n n n ne eut tral al Biog Bioge enic nic CO CO O O O O O - -neut n n n ne eut tra ral l CO CO O O O O O - -neut n n n ne eut tra ral l 2 2 2 2 2 2 2 2 2 waste liquid d d fuels waste liquid d d fuels liquid d d fuels Ci Cirrccu ular larr carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon technologies technologies technologies Electricity-based renewablehydrogen WE WE E- -Fuels ls: : Biog genic ic c- -based d sy y ynthec c fuels W W W- -Fuels ls: : Biogenic ic c- -based d synthec c fuels E E E- -Fuel ls s: : Electricit ty y y- -based d synthec c fuels with h h renewable e e e hydrogen Fig. 1. Chemical storage of biogenic and electricity-based renewable hydrogen in CO -neutral synthetic liquid fuels and integration into the natural carbon cycle technologies in this article, as they promote the closing of the availability and cost of renewable electricity. In terms the carbon cycle via the integration of the biogenic and/or of the utilization of renewable electricity for the mobility electricity-based renewable hydrogen. sector, E-Fuels thus stand in direct competition with elec- tric mobility. This competition also extends to other sec- tors such as the chemical and processing industries as 2 Comparative evaluation of E-Fuels, well as the building sector, which are anticipated to have an increasing demand for renewable electricity to support W-Fuels and WE-Fuels: case analysis their decarbonization efforts [22]. in Germany For W-Fuels and WE-Fuels, the amount of fuels produ- In the following paragraphs, E-Fuels, W-Fuels and WE-Fuels cible will be limited by the availability of biogenic waste. are comparatively evaluated in terms of feedstock potential, From the available amount of waste wood identified in product potential, demand for renewable electricity and asso- Germany above, ~1.5  million  Mg/year of W-Fuels in the ciated costs, efficiency as well as expected market relevance. form of kerosene, diesel or gasoline are producible. With the maximal integration of electricity-based renewable hydrogen, the producible amount doubles to ~3.0  mil- 2.1 Feedstock potential lion  Mg/year for WE-Fuels. To put this in perspective, With CO as the input material, there is theoretically no limit conventional oil-based sales in Germany in 2018 were to the amount of E-Fuels that can be produced. In the case 10.3  million  Mg/year kerosene, 18.1  million  Mg/year gas- of W-Fuels and WE-Fuels, to ensure that there is no food- oline and 37.5 million Mg/year diesel [35]. Bio-based CO - versus-fuel competition, it is essential that only biogenic neutral synthetic liquid fuels could thus contribute to waste and not biomass is used as the input for their produc- substituting for oil-based liquid fuels. tion. Generally, it would be most meaningful to make use of biogenic waste such as waste wood and forest residues that are either currently incinerated or not utilized for any pur - Results are based on preliminary process chain modelling with poses for W-Fuels production. In Germany, this potential is validated models in Aspen Plus. The process chain includes a ~6.9 million Mg/year for waste wood, and increases to 9.8 mil- fixed-bed slagging gasifier, cryogenic air-separation unit (only lion Mg/year when forest residues are included [32, 33]. W-Fuels), water scrubbing, water-gas-shift reactions (only W-Fuels), Rectisol sour gas cleaning, polymer-electrolyte- membrane electrolysis (only WE-Fuels), methanol synthesis, methanol-to-olefins synthesis, olefins oligomerization, 2.2 Product potential hydrocracking and hydrotreatment of liquid product. Please The limitation for E-Fuels production arises not from CO , refer to [34] for simplified flowsheets of gasification, primary which is theoretically unlimited, but from the availability syngas processing, CO-based methanol and methanol-to-olefins synthesis. and cost of renewable hydrogen, which are dependent on Recycling Ulizaon Photosynthesis Bio-H Bio-H Bio-CO C H Bio-CO x y 2 Recycling Ulizaon Photosynthesis Bio-H Bio-H C H Bio-CO x y 2 CO from atmosphere and/or biogenic pointsources C H Bio-CO x y 2 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 184 | Clean Energy, 2021, Vol. 5, No. 2 Table 1: Overall efficiency for technology routes for CO -neutral mobility Efficiency E-Mobility Hydrogen mobility E-Fuels W-Fuels Electricity production 100% 100% 100% – Hydrogen production – 75% 75% – Fuel synthesis – – 71% 50% Hydrogen liquefaction – 86% – – Distribution 95% 96% 99% 99% Fuelling 90% 97% 99% 99% Fuel cell – 60% – – Engine and drive train 85% 85% 40% 40% Overall efficiency 73% 31% 21% 20% Sources: Adapted from [37], W-Fuels based on [29]. feedstock. It thus optimizes the utilization of available bio- 2.3 Electricity demand and associated costs genic waste materials for the production of CO -neutral Of the three technology routes, E-Fuels have the highest synthetic liquid fuels. demand for renewable electricity. Hence, high electricity prices will translate into high production costs for E-Fuels. In Germany, subsidies for the expansion of renewable 2.4 Efficiency electricity have resulted in increasing electricity prices in the country. Today, Germany has one of the highest electri- An important evaluation criterion for the sustainability city prices in Europe, i.e. 19.57 EUR/MWh electricity in the and production costs is the degree of efficiency for the second half of 2019 (compared to the European average complete process chain from the raw material input, i.e. of 15.15 EUR/MWh for non-household consumers with all renewable electricity (for E-Fuels) and biogenic waste (for taxes and levies included [36]). This suggests that costs for W-Fuels and WE-Fuels), to the drive shaft of the vehicle. E-Fuels production will also be considerable, thus limiting Consider the example of CO -neutral diesel for heavy- the market competitiveness of E-Fuels compared to alter - load vehicles. E-Fuels and W-Fuels exhibit similar levels native CO -neutral synthetic liquid fuels. of overall efficiency at ~20% for the process chains span- In contrast, W-Fuels have very low demand for renew- ning from renewable electricity/biogenic waste to diesel able electricity. This is because biogenic waste provides its to vehicle engine (see Table 1). Depending on the degree own energy source—albeit only ~50%—for the fuel syn- of integration of renewable electricity, the overall effi- thesis. The remaining carbon in the biogenic waste that ciency of WE-Fuels will be between the levels of E-Fuels is not converted into syngas will be emitted as biogenic and W-Fuels. (i.e. CO -neutral) CO . Under the existing framework condi- At this point, it is important to note that the overall effi- 2 2 tions, the production costs for W-Fuels, at ~1 EUR/litre, will ciency of E-mobility is reported to be much higher (e.g. 73% be highly competitive [29]. for E-Lorry). However, this does not represent a suitable With the integration of renewable electricity into comparison for CO -neutral synthetic liquid fuels, as they W-Fuels, carbon loss in the form of biogenic CO can and E-mobility are based on different storage principles be avoided. WE-Fuels allow a flexible integration of (i.e. chemical storage versus electrochemical storage), ap- electricity-based renewable hydrogen. At the maximal plication areas and mobility sectors. An appropriate com- integration, WE-Fuels require only about half the re- parison would be with hydrogen mobility, which is also newable electricity that E-Fuels require. This is because based on a chemical-storage principle. Based on the ex- electricity-based renewable hydrogen is only required ample of a fuel-cell-powered lorry, an overall efficiency of to convert the remaining 50% of the carbon that would 31% is achievable via hydrogen mobility. While CO -neutral otherwise be emitted as biogenic CO in W-Fuels produc- synthetic liquid fuels exhibit lower efficiencies at ~20%, tion into additional syngas. Hence, we could say that, at this disadvantage in terms of efficiency must be weighed the maximal integration of electricity-based renewable against their significantly wider application possibilities. hydrogen, WE-Fuels—focusing on the energy balance— are made up of ~50% E-Fuels. This is associated with two significant advantages. First, due to the lower demand for 2.5 Expected market relevance electricity-based renewable hydrogen and thus renew- Under existing regulatory frameworks and conditions, able energy, the production costs for WE-Fuels will lie CO -neutral synthetic liquid fuels would have limited between that of W-Fuels and E-Fuels. Second, the full in- market relevance. To facilitate their market entry and suc- tegration of electricity-based renewable hydrogen would cessful implementation, the following regulatory changes double the producible quantities of WE-Fuels compared to will be necessary, namely (i) the accreditation of positive that of W-Fuels with the same amount of biogenic-waste Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 185 environmental impacts from CO-neutral synthetic liquid potential to contribute to substitution for conventional oil- fuels to fleet consumption, (ii) the accreditation of emis- based liquid fuels. While all three routes exhibited similar sions reduction from CO-neutral synthetic liquid fuels levels of overall efficiencies, E-Fuels are currently restricted to the required CO-reduction quota and (iii) financial (i.e. by the high cost and limited supply of renewable electricity tax) benefits for the utilization of CO-neutral synthetic in Germany. In comparison, CO -neutral synthetic liquid 2 2 liquid fuels. fuels generated from biogenic waste are largely independent Provided that the regulatory environment is favour - from the supply and cost of renewable electricity and can able, the market entry for W-Fuels is deemed the most rely on a constant supply of biogenic feedstock. Generally, promising in the short term due to the technology ma- under the prevailing regulatory and market conditions, turity of circular carbon technologies involved in W-Fuels CO -neutral synthetic liquid fuels will have limited market production, its comparatively low production costs as relevance and competitiveness. With supporting regulatory well as the wide range of application possibilities. The frameworks for their implementation, the market entry for size of a production plant for W-Fuels will be dependent W-Fuels is deemed most the promising in the short term in on the capacity of the biowaste preparation facility. At view of its maturity, lower production costs as well as the a production capacity of 100,000–200,000  Mg/year of wide range of application possibilities compared to those of W-Fuels, significant economies-of-scale effects will be other CO-neutral synthetic liquid fuels. possible. Through the stepwise integration of renewable elec- tricity, the W-Fuels technology route can be transformed Funding without additional efforts and costs—with the exception This research was funded by the German Federal Ministry of of the delivery of electricity-based renewable hydrogen— Education and Research (BMBF) through the research project grant for the production of WE-Fuels. Due to the possibility for no. 01LN1713A. All opinions, results and conclusions in the text flexible integration, the timing and degree of electricity- are those of the authors and do not necessarily reflect the opinion of the BMBF. based renewable hydrogen integration can be adapted to match fluctuations in availability/costs of hydrogen and market demands. In contrast to W-Fuels and WE-Fuels, the market rele- Conflict of Interest vance of E-Fuels will be strongly dependent on the avail- None declared. ability and affordability of electricity-based renewable hydrogen. In view of the anticipated competition for re- References newable electricity with E-mobility as well as with other carbon-intensive sectors for their decarbonization efforts, [1] European Commission. Questions and Answers: A  Hydrogen Strategy for a Climate Neutral Europe. 2020. https://ec.europa. E-Fuels are not expected to have a significant market share eu/commission/presscorner/detail/en/QANDA_20_1257 (12 in the near future. 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Hintergrundinformationen zur Infografik https://https://www.bmbf.de/files/bmwi_Nationale%20 Wasserstoffstrategie_Eng_s01.pdf (4 September 2020, date ‘Klimaneutral, erneuerbar, effizient: E-Lkw liegen vorn’. https://www.oeko.de/fileadmin/aktuelles/Hintergrund- last accessed). [23] Higman C, van der Burgt M.Gasification . 2nd edn. Burlington: Infografik-Wirkungsgrade.pdf (4 September 2020, date last accessed). Elsevier Science, 2011. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Clean Energy Oxford University Press

Chemical storage of hydrogen in synthetic liquid fuels: building block for CO2-neutral mobility

Clean Energy , Volume 5 (2) – Jun 1, 2021

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Copyright © 2021 National Institute of Clean-and-Low-Carbon Energy
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Abstract

Green hydrogen is anticipated to play a major role in the decarbonization of the mobility sector. Its chemical storage in CO -neutral synthetic liquid fuels is advantageous in terms of safety and reliability compared to other hydrogen storage developments, and thus represents a complementary building block to developments in electric and hydrogen mobility for the low-carbon transition in the mobility sector. Its development is especially relevant for transport sectors which will have no alternatives to liquid fuels in the foreseeable future. In this paper, three alternative technological routes for the chemical storage of hydrogen in CO -neutral synthetic liquid fuels are identified and comparatively evaluated in terms of feedstock potential, product potential, demand for renewable electricity and associated costs, efficiency as well as expected market relevance. While all three routes exhibited similar levels of overall efficiencies, electricity-based liquid fuels in Germany are currently limited by the high cost and limited supply of renewable electricity. In contrast, liquid fuels generated from biogenic waste have a constant supply of biogenic feedstock and are largely independent from the supply and cost of renewable electricity. Received: 12 November 2020; Accepted: 22 February 2021 © The Author(s) 2021. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com 180 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 181 Graphical abstract CO Biomass Biomass Biog Biogeenic nic CO CO O O O O O - -neutr n n n ne eut tral al Biog Bioge enic nic CO CO O O O O O - -neut n n n ne eut tra ral l CO CO O O O O O - -neut n n n ne eut tra ral l 2 2 2 2 2 2 2 2 2 waste waste liquid d d fuels liquid d d fuels liquid d d fuels Ci Cirrccu ular larr carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon technologies technologies technologies Electricity-basedrenewable hydrogen WE WE E- -Fuel ls s: : Biog geni ic c c- -based d synt y y hec c fuels W W W- -Fuel ls s: : Biogenic ic c- -based d synthec c fuels E E E- -Fuel ls s: : Electricit ty y y- -based d synthec c fuels with h h renewable e e e hydrogen Keywords: CO -neutral synthetic liquid fuels; renewable hydrogen; chemical storage; waste-to-fuels; electricity- based fuels; circular carbon technologies combination with fuel cells—as an integral component of Introduction its future fuel mix. Besides lowering the carbon footprint Hydrogen is a highly versatile element that can be used as in transport, it could contribute to improving energy effi- a feedstock, a fuel or an energy carrier and for storage. It ciency while at the same time reducing the EU’s depend- has numerous applications in diverse sectors ranging from ency on fossil imports, thus enhancing its energy-supply chemical production, metallurgy and energy to mobility, security [6]. In order to inform decision-makers about the and does not emit CO or pollute the environment when potential and future of hydrogen mobility, its prospects it is utilized [1]. Hence, it is anticipated to play a key role and challenges in the form of fuel-cell vehicles have been in the transformation of carbon-intensive sectors towards intensively investigated and reported in scientific litera- carbon neutrality. While hydrogen can be produced from ture (e.g. [7–10]). At the same time, alternative routes of almost all energy resources, it is currently predominantly hydrogen storage—e.g. in ammonia (NH), metal hydrides, generated from fossil fuels, i.e. natural gas and coal. Fossil- chemical hydrides, liquid organic hydrogen carriers such based hydrogen is generally referred to as ‘grey’ hydrogen as methylcyclohexane (MCH), carbohydrates or synthetic and its global production is associated with ~830 million hydrocarbons (e.g. [11–17])—have also garnered significant tons of CO per year. This is equivalent to the carbon foot- research attention. print of the UK and Indonesia combined [2]. In view of its A safe and reliable hydrogen-storage infrastructure has potential in supporting the decarbonization of industries been identified as a necessary enabling condition for the and mobility, global interest in the production, storage and public acceptance of hydrogen mobility [18]. In this con- utilization of ‘green’ hydrogen from renewable sources text, the chemical storage of green hydrogen in synthetic with a zero carbon footprint is thus growing steadily. hydrocarbons in the form of CO-neutral synthetic liquid A major sector in which green hydrogen is envisaged to fuels to substitute fossil-based transportation fuels for play a major role in decarbonization is mobility. Currently, road vehicles, aviation and shipping has a major advan- transportation accounts for approximately one-fifth of the tage compared to other hydrogen-storage developments. global CO emissions [3]. To complement the develop- This is because synthetic liquid fuels can be safely, easily ments in battery electric vehicles (e.g. [45 , ]) in lowering and reliably distributed via existing petrol station net- the CO emissions of the mobility sector, the European works as drop-in fuel or as fossil-fuel blends for use in Commission has identified green hydrogen—especially in conventional combustion engines [19]. This is not only an Ulizaon Photosynthesis Recycling Bio-H Bio-H Bio-CO C H Bio-CO x y 2 Recycling Ulizaon Photosynthesis Bio-H Bio-H C H Bio-CO x y 2 CO from atmosphere and/or biogenic pointsources C H Bio-CO x y 2 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 182 | Clean Energy, 2021, Vol. 5, No. 2 essential development for mobility sectors such as avi- via the Fischer–Tropsch processes or the two-stage syn- ation, shipping and road (freight) which will have no real thesis of gasoline/kerosene with methanol production in alternatives to liquid fuels in the foreseeable future [19]; it the first stage followed by methanol-to-gasoline/-kerosene also represents a complementary building block for elec- in the second stage—are implemented for fuel production. tric and hydrogen mobility for the road (passenger) sector. This technological route enables the chemical storage of The latter is especially relevant for existing/older road- renewable electricity via hydrogen in CO-neutral fuels. vehicle fleets as well as regions where new infrastructures This, however, is not the only technological route through for hydrogen carriers are not economically feasible. which CO -neutral synthetic liquid fuels can be produced. While growing interest in CO-neutral synthetic liquid Other alternatives also involve the chemical storage fuels has been observed in recent years, such develop- of hydrogen, albeit in bio-based CO-neutral synthetic ments have predominantly focused on electricity-based liquid fuels. fuels (e.g. [19, 20]). The potential of alternative routes has The prerequisite for the production of bio-based CO- been so far largely neglected. To address this gap, this art- neutral synthetic liquid fuels is the use of biogenic waste icle has two main objectives, namely (i) to identify alterna- materials (e.g. waste wood, forest residues) to deliver both tive technological routes for the chemical storage of green the carbon and hydrogen components required for the hydrogen in CO -neutral synthetic liquid fuels and (ii) the fuel production. Unlike CO in the case of E-Fuels, biogenic 2 2 comparative evaluation of the alternative technological waste materials not only act as a carbon source; they also routes in terms of feedstock potential, product potential, provide the chemical energy required for the fuel syn- demand for renewable electricity and associated costs, ef- thesis. Via waste gasification, the reaction between the ficiency as well as expected market relevance. biogenic waste and oxygen produces syngas  which con- We utilize a case  analysis approach for the compara- sists of a mixture of carbon monoxide, hydrogen and CO. tive evaluation to facilitate a quantification—as far as pos- Similarly to E-Fuels, syngas can be subsequently refined sible—of the parameters considered. Our investigation via different (i.e. direct or two-stage) synthesis technolo- focuses on Germany. In view of its adoption of a National gies for the production of synthetic fuels. We call such Hydrogen Strategy and the establishment of the National bio-based synthetic fuels W-Fuels—an abbreviation for Hydrogen Council to realize its ambition to become a ‘Waste-to-Fuels’ [28, 29]. As both carbon and hydrogen global leader in hydrogen technologies [21], Germany pre- components are biogenic in origin, W-Fuels produced this sents a rich case study for the analysis of the chemical way will be CO -neutral. W-Fuels thus represent a chem- storage of green hydrogen and its potential contribution ical storage for biogenic hydrogen in CO-neutral synthetic to CO -neutral mobility. Note that the intent of this art- liquid fuels. icle is not to carry out a life-cycle assessment of CO emis- During the W-Fuels production process, not all carbon sions of the different technological routes for the chemical contained in the biogenic waste materials will be con- storage of green hydrogen in CO-neutral synthetic liquid verted into W-Fuels. Carbon losses will occur along the fuels. It should be understood as a perspective article that process chain in the form of biogenic CO emissions. With presents a comparative evaluation of the diverse techno- the integration of hydrogen from renewable electricity logical routes along multiple parameters to sensitize into the process chain, such CO could be converted into readers about the potential and challenges of alternatives additional syngas [30, 31]. We call W-Fuels produced with to electricity-based synthetic fuels. the integration of additional electricity-based renewable hydrogen WE-Fuels, as they represent chemical storage for both biogenic and electricity-based renewable hydrogen 1 Alternative technology routes for in CO -neutral synthetic liquid fuels. WE-Fuels thus offer flexible chemical storage for electricity-based renewable the chemical storage of hydrogen in hydrogen, as its integration in the fuel synthesis is an op- CO -neutral synthetic liquid fuels tion and not—as in the case of E-Fuels—a must. Both carbon and hydrogen play essential roles in the pro- Fig. 1 illustrates the chemical storage of hydrogen in duction of CO-neutral synthetic liquid fuels. In recent CO -neutral synthetic liquid fuels in E-Fuels, W-Fuels years, the predominant focus by science, industry and pol- and WE-Fuels, and their integration into the natural itics has been placed on the production of electricity-based carbon cycle. Note that the technologies for conversion of fuels, i.e. E-Fuels, also known as ‘Power-to-X Fuels’ or ‘PTX- carbon and hydrogen sources into CO-neutral synthetic Fuels’. Generally, the carbon source for ‘classical’ E-Fuels liquid fuels are generally referred to as circular carbon is provided by CO from point sources and/or from the at- mosphere [19, 22]. CO , however, can not provide the chem- Please refer to [20] for an overview of E-Fuels production ical energy that is required for the fuel synthesis. Rather, processes. the chemical energy required is obtained from renewable Please refer to [23] and [24] for an overview of the gasification hydrogen that is generated via water electrolysis powered processes for syngas production. For an overview of the syn- by renewable electricity. Subsequently, standard synthesis thesis processes for fuel production from syngas, please refer to processes—either the direct synthesis of diesel/kerosene [25], [26] and [27]. Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 183 CO Biomass Biomass Biog Biogeenic nic CO CO O O O O O - -neutr n n n ne eut tral al Biog Bioge enic nic CO CO O O O O O - -neut n n n ne eut tra ral l CO CO O O O O O - -neut n n n ne eut tra ral l 2 2 2 2 2 2 2 2 2 waste liquid d d fuels waste liquid d d fuels liquid d d fuels Ci Cirrccu ular larr carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon Ci Cir rc cu ular lar r carbon ca arbon technologies technologies technologies Electricity-based renewablehydrogen WE WE E- -Fuels ls: : Biog genic ic c- -based d sy y ynthec c fuels W W W- -Fuels ls: : Biogenic ic c- -based d synthec c fuels E E E- -Fuel ls s: : Electricit ty y y- -based d synthec c fuels with h h renewable e e e hydrogen Fig. 1. Chemical storage of biogenic and electricity-based renewable hydrogen in CO -neutral synthetic liquid fuels and integration into the natural carbon cycle technologies in this article, as they promote the closing of the availability and cost of renewable electricity. In terms the carbon cycle via the integration of the biogenic and/or of the utilization of renewable electricity for the mobility electricity-based renewable hydrogen. sector, E-Fuels thus stand in direct competition with elec- tric mobility. This competition also extends to other sec- tors such as the chemical and processing industries as 2 Comparative evaluation of E-Fuels, well as the building sector, which are anticipated to have an increasing demand for renewable electricity to support W-Fuels and WE-Fuels: case analysis their decarbonization efforts [22]. in Germany For W-Fuels and WE-Fuels, the amount of fuels produ- In the following paragraphs, E-Fuels, W-Fuels and WE-Fuels cible will be limited by the availability of biogenic waste. are comparatively evaluated in terms of feedstock potential, From the available amount of waste wood identified in product potential, demand for renewable electricity and asso- Germany above, ~1.5  million  Mg/year of W-Fuels in the ciated costs, efficiency as well as expected market relevance. form of kerosene, diesel or gasoline are producible. With the maximal integration of electricity-based renewable hydrogen, the producible amount doubles to ~3.0  mil- 2.1 Feedstock potential lion  Mg/year for WE-Fuels. To put this in perspective, With CO as the input material, there is theoretically no limit conventional oil-based sales in Germany in 2018 were to the amount of E-Fuels that can be produced. In the case 10.3  million  Mg/year kerosene, 18.1  million  Mg/year gas- of W-Fuels and WE-Fuels, to ensure that there is no food- oline and 37.5 million Mg/year diesel [35]. Bio-based CO - versus-fuel competition, it is essential that only biogenic neutral synthetic liquid fuels could thus contribute to waste and not biomass is used as the input for their produc- substituting for oil-based liquid fuels. tion. Generally, it would be most meaningful to make use of biogenic waste such as waste wood and forest residues that are either currently incinerated or not utilized for any pur - Results are based on preliminary process chain modelling with poses for W-Fuels production. In Germany, this potential is validated models in Aspen Plus. The process chain includes a ~6.9 million Mg/year for waste wood, and increases to 9.8 mil- fixed-bed slagging gasifier, cryogenic air-separation unit (only lion Mg/year when forest residues are included [32, 33]. W-Fuels), water scrubbing, water-gas-shift reactions (only W-Fuels), Rectisol sour gas cleaning, polymer-electrolyte- membrane electrolysis (only WE-Fuels), methanol synthesis, methanol-to-olefins synthesis, olefins oligomerization, 2.2 Product potential hydrocracking and hydrotreatment of liquid product. Please The limitation for E-Fuels production arises not from CO , refer to [34] for simplified flowsheets of gasification, primary which is theoretically unlimited, but from the availability syngas processing, CO-based methanol and methanol-to-olefins synthesis. and cost of renewable hydrogen, which are dependent on Recycling Ulizaon Photosynthesis Bio-H Bio-H Bio-CO C H Bio-CO x y 2 Recycling Ulizaon Photosynthesis Bio-H Bio-H C H Bio-CO x y 2 CO from atmosphere and/or biogenic pointsources C H Bio-CO x y 2 Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 184 | Clean Energy, 2021, Vol. 5, No. 2 Table 1: Overall efficiency for technology routes for CO -neutral mobility Efficiency E-Mobility Hydrogen mobility E-Fuels W-Fuels Electricity production 100% 100% 100% – Hydrogen production – 75% 75% – Fuel synthesis – – 71% 50% Hydrogen liquefaction – 86% – – Distribution 95% 96% 99% 99% Fuelling 90% 97% 99% 99% Fuel cell – 60% – – Engine and drive train 85% 85% 40% 40% Overall efficiency 73% 31% 21% 20% Sources: Adapted from [37], W-Fuels based on [29]. feedstock. It thus optimizes the utilization of available bio- 2.3 Electricity demand and associated costs genic waste materials for the production of CO -neutral Of the three technology routes, E-Fuels have the highest synthetic liquid fuels. demand for renewable electricity. Hence, high electricity prices will translate into high production costs for E-Fuels. In Germany, subsidies for the expansion of renewable 2.4 Efficiency electricity have resulted in increasing electricity prices in the country. Today, Germany has one of the highest electri- An important evaluation criterion for the sustainability city prices in Europe, i.e. 19.57 EUR/MWh electricity in the and production costs is the degree of efficiency for the second half of 2019 (compared to the European average complete process chain from the raw material input, i.e. of 15.15 EUR/MWh for non-household consumers with all renewable electricity (for E-Fuels) and biogenic waste (for taxes and levies included [36]). This suggests that costs for W-Fuels and WE-Fuels), to the drive shaft of the vehicle. E-Fuels production will also be considerable, thus limiting Consider the example of CO -neutral diesel for heavy- the market competitiveness of E-Fuels compared to alter - load vehicles. E-Fuels and W-Fuels exhibit similar levels native CO -neutral synthetic liquid fuels. of overall efficiency at ~20% for the process chains span- In contrast, W-Fuels have very low demand for renew- ning from renewable electricity/biogenic waste to diesel able electricity. This is because biogenic waste provides its to vehicle engine (see Table 1). Depending on the degree own energy source—albeit only ~50%—for the fuel syn- of integration of renewable electricity, the overall effi- thesis. The remaining carbon in the biogenic waste that ciency of WE-Fuels will be between the levels of E-Fuels is not converted into syngas will be emitted as biogenic and W-Fuels. (i.e. CO -neutral) CO . Under the existing framework condi- At this point, it is important to note that the overall effi- 2 2 tions, the production costs for W-Fuels, at ~1 EUR/litre, will ciency of E-mobility is reported to be much higher (e.g. 73% be highly competitive [29]. for E-Lorry). However, this does not represent a suitable With the integration of renewable electricity into comparison for CO -neutral synthetic liquid fuels, as they W-Fuels, carbon loss in the form of biogenic CO can and E-mobility are based on different storage principles be avoided. WE-Fuels allow a flexible integration of (i.e. chemical storage versus electrochemical storage), ap- electricity-based renewable hydrogen. At the maximal plication areas and mobility sectors. An appropriate com- integration, WE-Fuels require only about half the re- parison would be with hydrogen mobility, which is also newable electricity that E-Fuels require. This is because based on a chemical-storage principle. Based on the ex- electricity-based renewable hydrogen is only required ample of a fuel-cell-powered lorry, an overall efficiency of to convert the remaining 50% of the carbon that would 31% is achievable via hydrogen mobility. While CO -neutral otherwise be emitted as biogenic CO in W-Fuels produc- synthetic liquid fuels exhibit lower efficiencies at ~20%, tion into additional syngas. Hence, we could say that, at this disadvantage in terms of efficiency must be weighed the maximal integration of electricity-based renewable against their significantly wider application possibilities. hydrogen, WE-Fuels—focusing on the energy balance— are made up of ~50% E-Fuels. This is associated with two significant advantages. First, due to the lower demand for 2.5 Expected market relevance electricity-based renewable hydrogen and thus renew- Under existing regulatory frameworks and conditions, able energy, the production costs for WE-Fuels will lie CO -neutral synthetic liquid fuels would have limited between that of W-Fuels and E-Fuels. Second, the full in- market relevance. To facilitate their market entry and suc- tegration of electricity-based renewable hydrogen would cessful implementation, the following regulatory changes double the producible quantities of WE-Fuels compared to will be necessary, namely (i) the accreditation of positive that of W-Fuels with the same amount of biogenic-waste Downloaded from https://academic.oup.com/ce/article/5/2/180/6245787 by DeepDyve user on 27 April 2021 Clean Energy | 185 environmental impacts from CO-neutral synthetic liquid potential to contribute to substitution for conventional oil- fuels to fleet consumption, (ii) the accreditation of emis- based liquid fuels. While all three routes exhibited similar sions reduction from CO-neutral synthetic liquid fuels levels of overall efficiencies, E-Fuels are currently restricted to the required CO-reduction quota and (iii) financial (i.e. by the high cost and limited supply of renewable electricity tax) benefits for the utilization of CO-neutral synthetic in Germany. In comparison, CO -neutral synthetic liquid 2 2 liquid fuels. fuels generated from biogenic waste are largely independent Provided that the regulatory environment is favour - from the supply and cost of renewable electricity and can able, the market entry for W-Fuels is deemed the most rely on a constant supply of biogenic feedstock. Generally, promising in the short term due to the technology ma- under the prevailing regulatory and market conditions, turity of circular carbon technologies involved in W-Fuels CO -neutral synthetic liquid fuels will have limited market production, its comparatively low production costs as relevance and competitiveness. With supporting regulatory well as the wide range of application possibilities. The frameworks for their implementation, the market entry for size of a production plant for W-Fuels will be dependent W-Fuels is deemed most the promising in the short term in on the capacity of the biowaste preparation facility. At view of its maturity, lower production costs as well as the a production capacity of 100,000–200,000  Mg/year of wide range of application possibilities compared to those of W-Fuels, significant economies-of-scale effects will be other CO-neutral synthetic liquid fuels. possible. Through the stepwise integration of renewable elec- tricity, the W-Fuels technology route can be transformed Funding without additional efforts and costs—with the exception This research was funded by the German Federal Ministry of of the delivery of electricity-based renewable hydrogen— Education and Research (BMBF) through the research project grant for the production of WE-Fuels. Due to the possibility for no. 01LN1713A. All opinions, results and conclusions in the text flexible integration, the timing and degree of electricity- are those of the authors and do not necessarily reflect the opinion of the BMBF. based renewable hydrogen integration can be adapted to match fluctuations in availability/costs of hydrogen and market demands. In contrast to W-Fuels and WE-Fuels, the market rele- Conflict of Interest vance of E-Fuels will be strongly dependent on the avail- None declared. ability and affordability of electricity-based renewable hydrogen. In view of the anticipated competition for re- References newable electricity with E-mobility as well as with other carbon-intensive sectors for their decarbonization efforts, [1] European Commission. Questions and Answers: A  Hydrogen Strategy for a Climate Neutral Europe. 2020. https://ec.europa. E-Fuels are not expected to have a significant market share eu/commission/presscorner/detail/en/QANDA_20_1257 (12 in the near future. 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Journal

Clean EnergyOxford University Press

Published: Jun 1, 2021

References